The servo system of known digital video recorders has a sophisticated clock control circuit, since control signals with precisely defined timing are required for coordination or coupling between the motor control, for switching the head amplifiers and for data exchange between the codec and the servo system. In order to define control signals of this type, a sensor supplies a position pulse PU, which forms the basis for the synchronization of the control signals, with every revolution of the scanner motor or of the head drum. Among other things, this position pulse PU of the scanner motor is used to carry out the synchronization with the incoming video pictures or video frames. Desired boundary conditions for the control signals in this case are freedom from jitter, synchronism, programmable phase offset and low real-time loading of the controller.
In order to fulfil the desired basic and special functions of a video recorder, in particular of a digital video recorder, it is at least necessary to make available and produce the following switching pulses:
a head switching signal HS1 for switching over head 1 and head 2, which is offset by 180.degree., PA1 a head switching signal HS2 for switching over head 3 and head 4, which is offset by 180.degree., PA1 a record/play changeover signal, and PA1 a synchronous track signal for alternative tracking control by means of the longitudinal track. PA1 a) detection of a position pulse PU of the scanner motor or of the head drum, which pulse is used for the definition of a time zero point to and determines the revolution period of the scanner motor and thus of the head drum, PA1 b) calculation of the time markers t.sub.1, t.sub.2, . . . t.sub.n of the control pulses, which are required for the following period, and of the bit patterns of the control signals for the corresponding time markers, PA1 c) storage of the time markers t.sub.1 -t.sub.n and of the bit patterns belonging to the corresponding time markers, PA1 d) parallel output of the corresponding bit pattern with the arrival of a corresponding time marker.
Both SD and HD recorders require flexible, controllable head switching signals, for example HS1 and HS2, which define the writing and reading range. In the case of four-head systems, the small spacing between the double heads of about 550 .mu.m means that a very small time interval between the two edges of the head switching signals must be possible, and with the said head spacing, this interval is approximately 55 .mu.s. In order to comply with-the specifications defined in the digital video codec standard (DVC standard), it is also necessary to set the beginning of writing with reference to the tape edge. In this case, it is possible to compensate for the mechanical tolerances by means of a programmable time delay between the sensor for producing the PU signal, which sensor is fitted on the head drum, and the switching-on edge of the head switching signal. This can both be achieved by a manual adjustment operation and be done automatically by means of a microcomputer or microcontroller. By evaluating specific information items on the tape and their temporal assignment, a time delay value is determined which normalizes the mechanical conditions of the respective device and thus meets the required DVC standard.
The record/play signal is likewise to be changed over synchronously in time with the beginning of the track or the beginning of the frame (beginning of the picture). In various special operating modes, such as assemble editing, caption editing and audio dubbing, data must be read and inserted at quite specific points on a track of the video tape. With reference to the beginning of the track, it is therefore necessary to produce a writing window which has precise timing and controls the writing of data.